1. Field of the Invention
The present invention relates to a method for producing L-threonine by fermentation, and particularly relates to a method for producing L-threonine with a microorganism of the genus Brevibacterium and Corynebacterium constructed by a gene recombination technique.
2. Description of the Prior Art
Hitherto, in order to render a wild strain capable of producing L-threonine from carbohydrates, it has been necessary to induce artificial mutants form the wild strain. There are many known L-threonine-producing artificial mutants. Most of the known threonine-producing mutants are resistant to .alpha.-amino-.beta.-hydroxy valeric acid (hereinafter referred to as "AHV"), and belong to the genus Brevibacterium or Corynebacterium. These microorganisms produce L-threonine in a yield of from 10 to 20%.
Examples of publications are U.S. Pat. No. 3,582,471, U.S. Pat. No. 3,580,810 and Japanese Publication No. 47-34956, in which threonine producing mutants resistant to AHV and belonging to the genera Brevibacterium, Escherichia and Corynebacterium are disclosed, respectively. Recent publications concering threonine production especially by mutant of the genera Brevibacterium and Corynebacterium are Japanese Published Unexamined Patent Application Nos. 51-54984, 53-101591, 54-32693, 54-35285, 54-35286, 54-35288, 54-37886 and 54-92692.
Another approach to increase the productivity of threonine in microorganism is found in U.S. Pat. No. 4,278,765 and Japanese Published Unexamined Patent Applications Nos. 55-131397 and 56-15696, in which threonine producing Escherichia coli strains transformed with a recombinant plasmid DNA and then constructed by a gene-recombination technique were disclosed.
However, it has been difficult to construct commercially applicable threonine-producer of Escherichia coli by the gene-recombination technique, because originally Escherichia strains can not express high productivity of L-threonine and then recombinant strains derived from such Escherichia strains can not produce high amount of L-threonine.
On the other hand, threre are many strains in the genera Brevibacterium and Corynebacterium which produce high amount of L-threonine, and therefore the strains of Corynebacterium and Brevibacterium may be suitable as the original strain for construction of L-threonine-producer by gene-recombination technique. However, although presence of plasmids in the strains of Brevibacterium and Corynebacterium is known (Publication of European Patent Application No. 0030391), the plasmids have no specific characteristics to be used as the marker for identification of the plasmids, and therefore it has been very difficult to select recombinant plasmids, derived from the plasmids of Brevibacterium and Corynebacterium. For the reason as above, it has been difficult to construct L-threonine-producer from the L-threonine producing Brevibacterium and Corynebacterium by gene-recombination technique.
A need therefore, continues to exist for development of novel process for production of L-threonine in high yields.